The invention relates to storage-stable one-component (1K) polyurethane prepregs and to shaped bodies produced therefrom.
1K polyurethane compositions are especially suitable for the production of storage-stable polyurethane prepregs and shaped bodies produced therefrom (composite components). The storage-stable polyurethane prepregs are obtainable by a process by means of impregnation in the form of weaves and scrims using these reactive polyurethane compositions.
Various molding processes, for example the reaction transfer molding (RTM) process, involve the introduction of the reinforcing fibers into a mold, the closing of the mold, the introduction of the crosslinkable resin formulation into the mold, and the subsequent crosslinking of the resin, typically by supplying heat.
One of the limitations of such a process is the relative difficulty in laying the reinforcing fibers into the mold. The individual layers of the weave or laid scrim have to be cut to size and matched to the different mold geometries. This can be both time-consuming and complicated, especially when the moldings are also to contain foam cores or other cores. Premoldable fiber reinforcement systems with easy handling and existing forming options would be desirable here.
Fiber-reinforced materials in the form of prepregs are already being used in many industrial applications because of their convenience of handling and the increased efficiency in processing compared to the alternative wet-layup methodology.
Industrial users of such systems, in addition to faster cycle times and higher storage stabilities—even at room temperature—are also demanding a way of cutting the prepregs to size, without contamination of the cutting tools with the often sticky matrix material in the course of automated cutting-to-size and laying-up of the individual prepreg layers.
As well as polyesters, vinyl esters and epoxy systems there are a number of specialized resins in the crosslinking matrix systems field. These also include polyurethane resins which, because of their toughness, damage tolerance and strength, are used particularly for production of composite profiles via pultrusion processes. A disadvantage often mentioned is that the isocyanates used are toxic.
Polyurethane composites also have superior toughness compared to vinyl esters, unsaturated polyester resins (UPE) or UPE-urethane hybrid resins.
Prepregs and composites produced therefrom that are based on epoxy systems are described, for example, in WO 98/50211, U.S. Pat. Nos. 4,992,228, 5,080,857, 5,427,725, GB 2007676, GB 2182074, EP 309 221, EP 297 674, WO 89/04335, U.S. Pat. Nos. 5,532,296 and 4,377,657, 4,757,120.
WO 2006/043019 describes a process for producing prepregs based on epoxy resin-polyurethane powders.
Furthermore, prepregs based on thermoplastics in powder form as a matrix are known.
US 2004/0231598 describes a method in which the particles are guided through a specific acceleration chamber with electrostatic charging. This apparatus serves for coating of glass substrates, aramid substrates or carbon fiber substrates for the production of prepregs made from thermoplastic resins. Resins mentioned are polyethylene (PE), polypropylene (PP), polyether ether ketone (PEEK), polyether sulphone (PES), polyphenyl sulphone (PPS), polyimide (PI), polyamide (PA), polycarbonate (PC), polyethylene terephthalate (PET), polyurethane (PU), polyesters and fluoropolymers. The thermoplastic prepregs textiles produced therefrom exhibit inherent toughness, good viscoelastic damping characteristics, unlimited storage stability, good chemical resistance and recyclability.
WO 98/31535 describes a method of powder impregnation, in which the glass fiber or carbon fiber strands to be impregnated are contacted with a particle/liquid mixture or particle/gas mixture in a defined speed profile. The powders consist here of ceramic or thermoplastic materials, including thermoplastic polyurethane.
WO 99/64216 describes prepregs and composites and a method for the production thereof where emulsions comprising polymer particles having sufficiently small dimensions to allow envelopment of individual fibers are used. The polymers of the particles have a viscosity of at least 5000 centipoise and are either thermoplastics or crosslinking polyurethane polymers.
EP 0590702 describes powder impregnations for production of prepregs, in which the powder consists of a mixture of a thermoplastic and a reactive monomer or prepolymer. WO 2005/091715 likewise describes the use of thermoplastics for production of prepregs.
Michaeli et al. describes the development of a powder methodology for a pultrusion process with thermoplastic polyurethanes, called TPUs, in Coatings & Composite Materials, No. 19, p 37-39, 1997. In addition, the article Processing and properties of thermoplastic polyurethane prepreg (Ma, C. C. M.; Chiang, C. L. Annual Technical Conference—Society of Plastics Engineers (1991), 49th 2065-9.) discloses thermoplastic polyurethane (TPU) prepregs based on solvents and water-containing TPU systems.
Prepregs having a matrix based on two-component polyurethanes (2-K PUR) are known. The 2-K PUR category essentially comprises the conventional reactive polyurethane resin systems. In principle, this is a system consisting of two separate components. While the critical constituent of one component is always a polyisocyanate, the critical constituent in the second component comprises polyols or in more recent developments also amino- or amine-polyol mixtures. The two parts are mixed together only shortly before processing. Thereafter, the chemical curing takes place through polyaddition with formation of a network of polyurethane or polyurea. Two-component systems have a limited processing time (operating time, pot life) after the mixing of the two constituents, since the onset of reaction leads to gradual viscosity increase and finally to gelling of the system. Many variables determine its effective processibility period: reactivity of the co-reactants, catalysis, concentration, solubility, moisture content, NCO/OH ratio and ambient temperature are the most important [Lackharze (Coating Resins), Stoye/Freitag, Hauser-Verlag 1996, pages 210/212]. The disadvantage of the prepregs based on 2-K PUR systems of this type is that only a short period of time is available for the processing of the prepreg to give a composite. Therefore, such prepregs are not storage-stable over a number of hours, let alone days.
There follows a description of the polyurethane prepregs or composites based on 2-K PUR systems. The article by K. Recker reports on the development of a 2-K polyurethane system for the resin mat process, with particular attention to the processing properties for SMC components (Baypreg-ein neuer POLYURETHAN-Werkstoff für das Harzmattenverfahren [A Novel Polyurethane Material for the Resin Mat Process], Recker, Klaus, Kunststoffe-Plastics 8, 1981).
WO 2005/049301 discloses a catalytically activated 2-K PUR system, wherein the polyisocyanate component and the polyol are mixed and processed by means of pultrusion to give a composite.
WO 2005/106155 discloses fiber-reinforced composites for the construction industry, which are produced by means of the long fiber injection (LFI) methodology with 2-K polyurethane systems.
JP 2004196851 describes composites which are produced from carbon fibers and organic fibers, for example hemp, using a matrix composed of 2-K PUR based on polymeric methylene diphenyl isocyanate (MDI) and specific compounds containing OH groups.
EP 1 319 503 describes polyurethane composites, wherein specific outer polyurethane layers are used for a fiber laminate which has been impregnated with a 2K PUR resin and encases a core layer (for example a paper honeycomb). The 2K PUR resin consists, for example, of MDI and a mixture of polypropylenetriols and diols of ethylene oxide-propylene oxide copolymers.
WO 2003/101719 describes polyurethane-based composites and the methods for production. These are 2-K polyurethane resins with defined viscosities and particular gel times.
2-K PUR systems are likewise discussed in: “Fiber reinforced polyurethane composites: shock tolerant components with particular emphasis on armor plating” (Ratcliffe, Colin P.; Crane, Roger M.; Santiago, Armando L., AMD (1995), 211 (Innovative Processing and Characterization of Composite Materials), 29-37.) and in Fiber-reinforced polyurethane composites. I. Process feasibility and morphology. (Ma, Chen Chi M.; Chen, Chin Hsing. International SAMPE Symposium and Exhibition (1992), 37 (Mater. Work. You 21st Century), 1062-74.)
Apart from the different binder basis, moisture-curing coating materials correspond to largely analogous 2K systems both in terms of composition and in terms of properties. In principle, the same solvents, pigments, fillers and auxiliaries are used. Unlike 2K coatings, for stability reasons, these systems do not tolerate any moisture at all before their application.
Also known are physically drying systems based on non-reactive PUR elastomers. These are linear thermoplastic urethanes of relatively high molecular weight, formed from diols and diisocyanates, preferably MDI, TDI, HDI and IPDI. Thermoplastic systems of this kind generally have very high viscosities and hence also very high processing temperatures. This is a crucial factor that makes them difficult to use for prepregs. In the production of prepregs with fiber composites, the use of powders in reactive systems is comparatively unusual and has to date been restricted to a few fields of use. Probably the most common method of transferring a powder to a fiber surface is the fluidized bed impregnation method. A flow directed upward puts powder particles in a state in which they have fluid-like properties. This method is employed in EP 590 702. In this case, the strands of individual fiber bundles are teased apart and coated with the powder in the fluidized bed. The powder consists here of a mixture of reactive and thermoplastic powder, in order thus to optimize the properties of the matrix. Individual rovings (fiber bundles) are finally laid together, and several plies are compressed at a pressure of 16 bar for about 20 minutes. The temperatures vary between 250 and 350° C. Frequently, however, there is irregular coating in the fluidized bed method, especially when strands are not pulled apart.
In this regard, US 20040231598 presents a method which works in a similar way to the fluidized bed method. In this case, an air stream transports the particles to the substrate and there is homogeneous deposition of the powder by virtue of a specific construction.
A further process is described by US 20050215148. In that case, with the apparatus just mentioned, homogeneous distributions of the powder on the fiber are achieved. The particle size ranges here from 1 to 2000 μm. Coating in a multitude of experiments is effected from one or two sides. The homogeneous application of the powder, after subsequent pressing of the prepregs, produces laminates without air inclusions.
A further application, WO 2006/043019, describes the use of epoxy- and amino-terminated resins in powder form. The powders here are mixed and applied to the fibers. Subsequently, the particles are attached by sintering. The particle size is between 1 and 3000 μm, but preferably between 1 and 150 μm.
This restriction in the particle size to comparatively small diameters is also recommended in a study from Michigan State University. The theory here is that particles having small diameters are more likely to be able to penetrate into cavities between individual filaments than particles having high diameters (S. Padaki, L.T. Drzal: a simulation study on the effects of particle size on the consolidation of polymer powder impregnated tapes, Department of Chemical Engineering, Michigan State University, Composites: Part A (1999), pp. 325-337).
As well as in prepreg methodology, reactive powder systems are also used in other conventional methods, for example in winding methodology [M. N. Ghasemi Nejhad, K. M. Ikeda: Design, manufacture and characterization of composites using on-line recycled thermoplastic powder impregnation of fibers and in-situ filament winding, Department of Mechanical Engineering, University of Hawaii at Manoa, Journal of Thermoplastic Composite Materials, Vol 11, pp. 533-572, November 1998] or in the pultrusion method. For the pultrusion method, for example, fiber ropes (towpregs) are coated with the powder and first wound and stored in the form of what are called towpregs. One means of production is described in an article in the SAMPE Journal [R. E. Allred, S. P. Wesson, D. A. Babow: powder impregnation studies for high temperature towpregs, Adherent Technologies, SAMPE Journal, Vol. 40, No. 6, pp. 40-48, November/December 2004]. In a further study, towpregs of this kind were pressed together and cured by the pultrusion process to give material components [N. C. Parasnis, K. Ramani, H. M. Borgaonkar: Ribbonizing of electrostatic powder spray impregnated thermoplastic tows by pultrusion, School of Mechanical Engineering, Purdue University, composites, Part A, Applied science and manufacturing, Vol. 27, pp. 567-574, 1996]. Even though the production of towpregs and subsequent compression in the pultrusion process has already been conducted with thermoset systems, only thermoplastic systems for the most part have been used to date in this process.
Storage-stable polyurethane prepregs and shaped bodies produced therefrom are known from DE 102009001793 and DE 102009001806. DE 102009001793 and DE 102009001806 describe a method for production of storage-stable prepregs, essentially composed of A) at least one fibrous carrier and B) at least one reactive pulverulent polyurethane composition as matrix material, which consist of at least one polyurethane component and at least one resin component. DE 102010029355 provides for the melt application of polyurethane prepreg systems; DE 102010030233 describes meandering polyurethane prepreg systems. DE 102010030234 details solvent-containing polyurethane prepreg systems. DE 102010041239 claims coloured polyurethane prepreg systems. DE 102010041256 in turn concerns polyurethane prepreg systems on fixed films, while DE 102010041243 describes polyurethane prepreg systems having a proportion by volume of fibers of less than 50%. DE102011006163 describes the use of a liquid resin component.
What is common to all these documents is that the matrix formulations described therein consist of a polyurethane component and a resin component.